Copper

Copper (Cu) is an essential nutrient to all higher plants and animals. In animals, it is found primarily in the bloodstream, as a cofactor in various enzymes, and in copper-based pigments. In sufficient amounts, copper can be poisonous or even fatal to all organisms.

The level of free copper in water that can kill varies enormously across species. Small invertebrates like shrimps or snails only require a tiny amount.

Care should be taken when adding stones or rock you've collected yourself as it may contain copper ore. Never add ornaments with copper in them.

Copper is often present in tap water due to the common use of copper pipes (especially in old houses (say pre1970s). To reduce levels of dissolved copper, run the water for 2–5 minutes before collecting it for use in your aquarium. That said, most municipal water supplies are intentionally made moderately hard and alkaline, so as not to dissolve copper pipes and the lead solder that used to used to join them.

Poly-filter foam can be used to remove copper from water. The foam will turn green if it has absorbed copper.

Copper is used in anti-fungus/bacteria/snail treatments (typically copper sulphate), so if you have freshwater shrimps in your tank, great care should be taken to prevent the copper in these treatments killing them.

Water conditioners can render this metal harmless by chelating the metal.
Chelated copper is only 1% as toxic as non-chelated (free) copper.

Dangerous level of copper for shrimps is 0.03 mg per litre.

Dangerous level of copper for algae and bacteria is 0.08 mg per litre.

Dangerous level of copper for some fish, snails and plants is 0.10 mg per litre.

Copper sulphate is a common treatment for infections in marine aquaria (ones without invertebrates, at least), since there is already plenty of salt, the common remedy in fresh water.